two novel compact triple-band microstrip annular … · better impedance matching and harmonic...

Progress In Electromagnetics Research Letters, Vol. 5, 87–98, 2008

TWO NOVEL COMPACT TRIPLE-BAND MICROSTRIPANNULAR-RING SLOT ANTENNA FOR PCS-1900 ANDWLAN APPLICATIONS

H. Sabri and Z. Atlasbaf

Faculty of Engineering, Department of Electrical EngineeringTarbiat Modares University (TMU)Tehran, Iran

Abstract—This paper presents two compact ring slot antennaswhich are suitable for the PCS-1900 and the 2.4/5-GHz triple-bandoperations. The first antenna consists of three annular ring slots.The outer ring is responsible for exciting the first resonant modewhereas the middle ring excites the second resonant mode. Theinner most rings, through their Y-shaped slots, create a wide upperoperating band by combining the third and fourth resonant modes. Toimprove this antenna, we have employed circular Photonic Bandgap(PBG) structures in order to obtain a smaller slot antenna withbetter radiation characteristics. In this design, the cross-polarizationlevel in the E-plane has reduced compared to the first antenna by5.5 dB, 0.3 dB and 4 dB in the three resonant bands. Also, the cross-polarization in H-plane has reduced by an amount of 3 dB. In addition,the obtained results show that the co-polarization patterns are verysimilar in all three frequency bands. In both cases we have reducedthe size of antennas to 56% and 42% respectively, of conventionalmicrostrip slot antennas. The simulation results are verified bymeasurements.

1. INTRODUCTION

The rapid progress in wireless communications requires the develop-ment of lightweight, low-profile, flush-mounted and single-feed anten-nas. Also, it is highly desirable to integrate several RF modules fordifferent frequencies into one piece of equipment. Hence, multi-bandantennas that can be used simultaneously in different standards havebeen in the focus points of many research projects [1, 12–19]. Amongthese standards, the following frequency bands can be mentioned:

88 Sabri and Atlasbaf

(1) PCS-1900 requires a band of 1.85–1.99GHz;(2) IEEE 802.11b/g requires a band of 2.4–2.484 GHz;(3) IEEE 802.11a requires a band of 5.15–5.35 GHz and an additional

band of 5.725–5.825GHz;(4) HiperLAN2 requires a band of 5.47–5.725 GHz besides the band

of 5.15–5.35 GHz.

Microstrip antennas are very attractive because of their low profile,low weight, conformal to the surface of objects and easy production.

A large number of microstrip patches to be used in wirelessapplications have been developed. Various shapes such assquare, rectangle, ring, disc, triangle, elliptic, pentagonal, kite-like, etc. have been introduced [2–5]. In comparison to patchelements, the antennas with slot configurations demonstrate enhancedcharacteristics, including wider bandwidth, less conductor loss andbetter isolation. Particularly, the multi-ring structure is a versatileapproach for multi-band and broadband design. Also, feeding thesestructures could be simpler by using suitable line to ring techniquesfor circular slots.

Recently, researchers have become more interested in reducing thesize of microstrip slot antennas to make them small enough for wirelessterminal applications. Utilizing shorting pin, high dielectric constantmaterial, and some novel techniques in feeding are some beneficialways to get reduced sized antenna. Furthermore, PBG can be usedin reducing the size of antennas. These structures which have somespecific characteristics such as ability to suppression of surface waves,improving the radiation performance, and reducing the size of antennaare very attractive to use. In addition, they have been utilized in somepapers [6–11].

In [9], a fork like PBG structure has been used to reduce thesize of antenna. In [10], a PBG structure is used in order to achievebetter impedance matching and harmonic suppression of microstrippatch antenna. In [11] it is shown that by employing PBG structureswe can get an antenna with lower cross-polarization.

In this paper, we present a simple compact design of a triple-frequency annular-ring slot antenna, fed by a simple microstrip-line.The size of the presented antenna can be greatly reduced by embeddinga pair of symmetric Y-shaped slots in the center circular patch. In thefrequency range of interest, the presented antenna has four resonantmodes, while the antenna without any embedded slots possesses hasonly three resonant modes. In the second design we have tried tomake this antenna more compact and efficient by utilizing circularPBG structures. The very similar co-polarization radiation patterns

Progress In Electromagnetics Research Letters, Vol. 5, 2008 89

differs our proposed antennas from previously presented multi-bandantenna [12–18]. Also, the E-plane cross-polarization level of thesecond design is reduced by about 5.5 dB, 0.3 dB and 4 dB in the threeresonant bands compared with the first design. Also, H-plane cross-polarization level of the second design has been reduced 3 dB in thethree resonant bands.

2. THE GEOMETRY OF THE ANTENNA

Figure 1 shows our proposed antenna. It consists of three annularring slots etched in the ground of a dielectric substrate with a size ofA × B, a permittivity of εr, thickness of h and loss tangent of tan δ.The diameter of the outer ring is D1 where t1 is its width. The middleannular ring has a diameter of D2 and a width of t2.

Figure 1. Geometry of the proposed triple-frequency annular-ringslot antenna.

A pair of Y-shaped slots is etched in the middle of the structure.Their diameters are D3, width w and α as their opening angle. Theseslots are connected to the middle ring slot via a straight slot oflength ln. The rings are excited by an open ended 60 Ω microstriptransmission line of length L. This line is connected to a 50 Ω line by


a tapered line with length tap. Good matching can be achieved bychanging the length L.

The same figure can also be applied for the second design exceptfor the circular PBG structures on the substrate. In [6], a PBGstructure has been used to control harmonic frequencies of annularslot antennas and some formulas have been suggested for decreasingthe level of these harmonic frequencies. During our researches we havedistinguished that some restrictions exist in determining the number ofPBG structures. Consider “a” and λs as a distance between elementsof PBG structures and wavelength in the slots, respectively. In afrequency band which we want to perform miniaturization a range forthe ratio a/λs is optimal: 0.05 < a/λs < 0.15. Also, we should specifya relation between “a” and “r” (the radius of PBG structures’ element)which should be 0.1 < r/a < 0.2 for an optimal design. According tothese limitations, we have chosen two PBG structures which have eightelements. These are shown by dashed circles in Figure 1 with radiir1 and r2 respectively. The dimensions of the first antenna (withoutPBG structures) and the second one (with PBG structures) are shownin Table 1.

Table 1. Specifications of proposed antennas on a substrate (TaconicRF-35) with h = 0.762 mm, εr = 3.5 and tan δ = 0.0018. Alldimensions are in mm except α which is indicated in degrees.

A B D1 D2 D3 t1 t2 w ln L tap r1 r2

Antenna 1 42.5 42 36 23.4 12 1 1 0.5 4.2 11.7 0.4 143 - -

Antenna 2 38.5 38 34.4 22.6 10.6 1 1 0.5 5 9.7 0.4 145 1.5 1.1

α

3. NUMERICAL RESULTS

Figure 2 shows the return loss of the first antenna obtained by HFSSas a function of frequency for five different opening angles (α). Itshould be mentioned that α = 0◦ case corresponds to no Y-shapedring slots, by being left only with the horizontal slots of length ln. Inthis case, the center frequencies of the second and third modes make awide VSWR≤ 2 impedance bandwidth. As the angle (α) increases, thesecond and the third resonant modes are shifted to lower and higherfrequencies respectively, resulting in a wider upper bandwidth thanthat in the previous case (α = 0◦).

By optimizing the angle α, we can obtain the most appropriateresults. Figure 3 shows the experimental and simulation resultsobtained for α = 143◦. As can be seen, the proposed antenna is suitable


Figure 2. Return losses of the first antenna for five different openingangles (α) [deg].

Figure 3. Return loss of the first antenna; measurement andsimulation (α = 143◦).

for PCS-1900, the 2.4 GHz (2400–2484 MHz) and 5 GHz (5–5.9 GHz)commercial frequency bands.

Figure 4 shows simulated and measured return loss of the secondantenna. The discrepancies between the measured data and theobtained results by HFSS could be due to inaccuracies in printing theY-shaped slots, which are in the ground plane of the second antenna.It has shown in Figure 2 that the second resonant band is dramaticallysensitive to the change of α.

The experimental results prove that the antenna with PBGstructures occupies less area than the other one without PBGstructures. Our first antenna needs 33% and 56% of the area ofthe antennas presented in [17] and [18] respectively, if they scale


Figure 4. Return loss of the second antenna; measurement andsimulation (α = 145◦).

to cover the 1.9 GHz band on a substrate having εr = 3.5 but thesecond antenna only needs 25% and 42% area of antennas in [17]and [18] respectively. Furthermore, it is found that the first and thesecond operating frequencies of the proposed antennas approximatelycorrespond to the circumference of the annular-ring slot being about0.73λs ∼ 0.92λs. It is also noted that the wavelength in the slot λs isdetermined to be about 0.78 free-space wavelengths from [18].

4. RADIATION CHARACTERISTICS

The far-field patterns of the proposed antennas in both the x-y (H-plane) and z-y (E-plane) planes at the three resonant-bands wereobtained by Ansoft HFSS software. As shown in Figures 5 and 6,the patterns for both antennas have very low cross-polarization (theco- and cross-polarized fields correspond to Eθ and Eφ, respectively)levels and the three frequencies have the very same polarization planesthat differ our presented antennas from latest developed multi-bandantennas [12–18].

The cross-polarization level in the E-plane at the lower band ofantenna without PBG is relatively high, which is expected to be dueto the diffractions from the edges of the small ground plane. Thiscross-polarization level can be decreased by enlarging the ground planesize. By changing the ground plane dimensions from 42.5 × 42 mm2

to 46.5 × 46 mm2, the cross-polarization level decreases from −7 dBto −16 dB. The antenna with PBG structures has a better cross-polarization level in all three bands. As can be seen from Figures 5 and6, the cross-polarization levels of E-plane of the second antenna have


Figure 5. Measured and simulated E & H-plane radiation pattern ofthe first antenna (a) at 1.95 GHz, (b) at 2.44 GHz, (c) at 5.5 GHz.


Figure 6. Measured and simulated E & H-plane radiation pattern ofthe second antenna (a) at 1.9 GHz, (b) at 2.44 GHz, (c) at 5.5 GHz.


been decreased 5.5 dB, 0.3 dB and 4 dB compared to the first antennafor three resonant-bands respectively and the cross-polarization levelof H-plane of second antenna for all resonant-bands is reduced byan amount of 3 dB. It can be observed from radiation patterns ofthese antennas that they are suitable for triple band applications. Thegains of the first antenna in three resonant-bands are 0.5 dBi, 1.5 dBiand 4.3 dBi respectively. The gains of second antenna in its threeresonant-bands are 3.1 dBi, 0.8 dBi and 0.1 dBi respectively. It shouldbe mentioned that the measurement of the second resonant-band centerfrequency of antenna with PBG structure has been performed at2.37 GHz instead of 2.44 GHz.

Figure 7(a) shows a photograph of the proposed antenna withoutPBG structures and Figure 7(b) shows the second one with PBGstructures.

(a)

(b)

Figure 7. Photograph of the presented antennas. (a) First antenna.(b) Second antenna.


5. CONCLUSION

In this paper, two microstrip-line-fed annular-ring slot antennas withcenter patches embedded by Y-shaped slots have been proposed forPCS-1900, 2.4/5-GHz triple-band applications. The second antenna isthe enhanced version of the first one. The improvement in radiationpattern achieved by utilizing PBG structures and this method has ledto obtain a smaller antenna than the first one. The substrate area ofthe presented antenna without PBG structures is reduced more than44% compared with that of a conventional annular-ring slot antenna.This reduction is more than 58% for the second antenna with PBG.Both of them have demonstrated very similar copolarization radiationpatterns and very low cross-polarization levels in all three operatingbands. By employing PBG structures, the second antenna presented5.5 dB, 0.3 dB and 4 dB lower cross-polarization level in the E-planeof three resonant-bands respectively, and 3 dB lower cross-polarizationlevel in the H-plane of all three resonant-bands compared to the firstdesign.

ACKNOWLEDGMENT

The authors wish to acknowledge Iran Telecommunication ResearchCentre (ITRC) for partial financial support of this study on contractNo. TMU 87-06-32.

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two novel compact triple-band microstrip annular … · better impedance matching and harmonic...

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